1. Field of the Invention
The invention concerns plastic articles, the surfaces of which bear microstructure, such as retroreflective cube-corner sheeting, Fresnel lenses, total internal reflecting films, intermeshable articles, information carrying discs, and the like. The invention also concerns an improved method of making such plastic articles.
2. Description of the Related Art
U.S. Pat. No. 3,689,346 (Rowland) teaches a process for the continuous replication of retroreflective cube-corner sheeting by depositing a crosslinkable, partially polymerized resin on a master negative molding surface and employing actinic light or heat to solidify the resin, thus replicating the surface. The resins used typically exhibit relatively high levels of shrinkage upon solidifying or curing, thus giving rise to optical imperfections in the cube-corner microstructure.
U.S. Pat. No. 4,576,850 (Martens) states, "An article comprising crosslinked polymer with hard and soft segments or moieties having a microstructure-bearing surface is prepared by a process comprising filling a mold master, bearing or encoded with microstructure to be replicated, with a fluid, castable, one-part, preferably solvent-free, radiation addition-polymerizable, crosslinkable, organic oligomeric composition (or precursors thereof) having `hard` segments and `soft` segments, exposing the resulting cast compositions to radiation, preferably actinic radiation such as ultraviolet radiation, and thereby forming said articles, e.g., a retroreflective cube-corner sheeting, Fresnel lens or video disc." (See Abstract). In addition to the hard ("H") and soft ("S") segments, the composition preferably includes one or more "E" precursors "containing a radiation sensitive addition-polymerizable, functional group such as acrylyl, methacrylyl, allyl or vic-epoxy group" (col. 4, 1s. 63-66). The composition may also contain diluting monomers which "are addition-polymerizable monomers, viz., ethylenically unsaturated monomers and vic-epoxy reactive diluents. The diluting monomers contribute to the `H`, `S` or `E` content of the oligomeric composition" (col. 36, 1s. 10-12). The composition may include an addition-polymerization catalyst, preferably a photopolymerization catalyst at about 0.25 to 1.0% of the oligomeric composition.
Almost every example of the Martens patent tests the ability of the oligomeric composition to replicate a diffraction grating master or a video disc master (Example 22), both of which have fine microstructure. The exceptions are Example 20, which reports the replication of cube corners 0.128 mm in depth, and Example 21, which reports the replication of a Fresnel lens, both of which have relatively large microstructures, i.e., several magnitudes greater than that of the diffraction grating. Because a curable resin composition shrinks when cured, it can be more difficult to replicate with precision relatively large microstructures like those of Martens' Examples 20 and 21, as compared to the fine microstructures of the masters used in other examples.
In Example 20 of Martens, a platen press at 70.degree. C. forced the curable mixture into the cube corner depressions, and the thickness of the cured oligomeric resin was 0.25 mm, nearly twice the 0.128 mm depth of the microstructure. In Example 21, the oligomeric resin composition was applied at a thickness substantially in excess of the depth of the Fresnel lens elements in order to result in a self-supporting Fresnel lens after the polyester sheet was stripped off.
U.S. Pat. No. 4,374,077 (Kerfeld) specifically concerns information carrying discs such as laser readable video discs which, as noted above, have a fine microstructure. Kerfeld teaches that these discs "can be deposited between a stamper and a substrate to which the polymerized mass is to adhere. The photopolymerizable mass is deposited by moving a bead or wave of the photopolymerizable liquid between a flexible substrate or flexible stamper and the corresponding optical disc stamper or substrate so as to fill the pattern in the stamper and then irradiating the photopolymerizable mass, through either the stamper or the substrate . . . . The advancement of the bead of liquid by the substrate being rolled out over the master has been found to remove substantially all of the air in the mold. The leading edge of the bead tends to pick up and carry the air without re-depositing it in progressive features of the master because the air bubbles rupture, releasing the gas . . . . As long as the substrate or master is able to bend sufficiently to enable the formation of a bead and propagation of that bead as the bend-point is moved forward, the substrate or master is sufficiently flexible" (col. 2. lines 3-68). Substrates used in the Kerfeld examples are flexible films of polyester, poly(vinyl chloride), and poly(methyl methacrylate).
U.S. Pat. No. 4,414,316 (Conley) concerns plastic articles, the surfaces of which replicate relatively large microstructures such as Fresnel lenses. In Examples 1 and 3, a polyester film is coated with a UV-curable resin that is pressed against a lenticular pattern and cured by directing ultraviolet radiation through the polyester film while the resin is in contact with the molding surface. FIG. 2 shows that in the resulting products, the thickness of a layer 12 of cured oligomeric resin is large compared to the depth of lenticular formations 13. The UV-curable resin preferably is an acrylate urethane polyester oligomer. See also U.S. Pat. No. 4,420,502 (Conley).